Refactor low-level crypto to C++ style.

[ni4]

This is rather huge PR, which would do simple things:

• avoid plain C structs and functions by replacing those with C++ classes/methods
• use references instead of pointers
• avoid static fixed-size buffer in favor of std::vector
• use C++ wrappers with destructors for proper Botan/OpenSSL objects deallocation.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.69332% with 195 lines in your changes missing coverage. Please review.

Project coverage is 84.86%. Comparing base (3bd1b71) to head (32a11db).

Files with missing lines	Patch %	Lines
src/lib/crypto/rsa_ossl.cpp	75.80%	45 Missing :warning:
src/lib/crypto/sm2.cpp	77.67%	25 Missing :warning:
src/lib/crypto/elgamal.cpp	77.50%	18 Missing :warning:
src/lib/crypto/dsa.cpp	81.39%	16 Missing :warning:
src/lib/crypto/dsa_ossl.cpp	82.97%	16 Missing :warning:
src/lib/crypto/ecdh.cpp	84.94%	14 Missing :warning:
src/lib/crypto/rsa.cpp	86.86%	13 Missing :warning:
src/lib/crypto/ec.cpp	75.55%	11 Missing :warning:
src/lib/crypto/ecdsa.cpp	85.71%	9 Missing :warning:
src/lib/crypto/eddsa.cpp	84.21%	9 Missing :warning:
... and 7 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2292 +/- ## ========================================== + Coverage 84.81% 84.86% +0.05% ========================================== Files 116 116 Lines 23292 22986 -306 ========================================== - Hits 19755 19507 -248 + Misses 3537 3479 -58 ```

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